It has been known for some time that a hot electron transistor, especially a ballistic transistor, could potentially be operated at frequencies in excess of those achievable with conventional (diffusive) transistors. See, for instance, T. E. Bell, IEEE Spectrum, February, 1986, pp. 36-38, incorporated herein by reference. Various types of hot electron transistors (HET) have been proposed. This application is concerned with one particular class of HETs, namely, compound semiconductor HETs. For a brief review, see L. F. Eastman, ibid, pp. 42-45, also incorporated herein by reference.
Essentially all the prior art work on compound semiconductor HETs has been concerned with GaAs-based devices, and recently ballistic transport has indeed been observed in GaAs-based research devices. A. F. J. Levi et al, Physical Review Letters, Vol. 55(19), pp. 2071-2073; M. Heiblum et al, Physical Review Letters, Vol. 55(20), pp. 2200-2203 and M. I. Nathan et al, IEEE Spectrum, February 1986, pp. 45-47.
The ballistic transport was observed in devices having geometries that are thought to hold promise for implementation as a practical HET. The first type, frequently referred to as a planar doped barrier transistor (PDBT, see for instance, J. R. Hayes et al Electronic Letters, Vol. 20(21), pp. 851-852) or "camel" transistor, J. M. Shannon, IEEE Proceedings, Vol. 128(9), pp. 134-140 (1981), both incorporated herein by reference) uses thermionic injection and comprises emitter, base, and collector, with an appropriately shaped potential barrier between emitter and base, and a second barrier between base and collector. The second type, which is referred to as a tunneling hot electron transfer amplifier (THETA, see for instance, M. Heiblum, Solid State Electronics, Vol. 24, pp. 343-366) differs from the first type in having tunnel injection into the base. Both of the above types are unipolar; however, bipolar HETs have also been proposed in GaAs/AlGaAs.
The flow of electrons from emitter to base is controlled in both types by varying the emitter/base barrier potential by means of an applied voltage V.sub.eb. Similarly, the flow of electrons from the base to the collector can be controlled by means of an externally applied voltage V.sub.bc between base and collector. Under normal operating conditions, V.sub.bc reverse biases the base/collector junction. Electrons injected from the emitter into the base have energy substantially greater than the thermal energy of the ambient electrons in the base. These "hot" electrons ideally traverse the base without undergoing significant scattering. If the base/collector barrier is caused to be lower than the hot electron energy then some of the hot electrons can cross the barrier, be transmitted through the depletion region of the collector, and enter the sea of conduction electrons in the collector.
As will be readily understood by those skilled in the art, various difficulties have to be overcome before a device of this type can function as a practical HET. Among these difficulties is quantum mechanical reflection of the hot electrons by the base/collector barrier, and space charge limited current. Probably the greatest obstacle, however, is the difficulty of achieving substantially complete hot electron transport through the base, and, desirably, also through the depletion region of the collector.
A recently filed U.S. patent application Ser. No. 871,494, titled "Hot Electron Transistor", filed June 6, 1986 by J. R. Hayes et al), incorporated herein by reference, discloses means for achieving improved HETs. Among the means are use of a direct bandgap compound semiconductor material in the "transit" region of the HET, with the conduction electrons in the transit region material having relatively small effective mass, advantageously substantially smaller than the conduction electron effective mass in GaAs. Furthermore, the transit region comprises a compound semiconductor material in which the total scattering rate of the hot electrons is relatively small, advantageously substantially less than the scattering rate would be in GaAs of identical ambient electron resistivity. Exemplary of such materials are InAs, InSb, InGaAs, HgCdTl, and PbSnTe.
However, even though HETs that incorporate the teachings of the U.S. Ser. No. 871,494 patent application can attain performance levels that are substantially better than those obtainable with a GaAs-based HET of the same geometry, the performance levels are in many cases still inadequate for practical HETs. For instance, prior art HETs are generally incapable of producing substantial current gain (e.g., .beta.&gt;10, where .beta. is the common emitter current gain) at room temperture (300 K.), and typically need to be operated at low temperatures (e.g. 77 K.).
In view of the operational simplification that results from the possibility of room temperature operation of a HET, and of the general desirability of improved characteristics such as .beta., means for achieving further improvements in HET characteristics would be of considerable significance. This application discloses such means.